In a gravel pack operation, a service seal unit mounted on a work string is reciprocated relative to certain flow ports and sealing surfaces within a packer bore to route service fluid along various passages. The service seal unit carries vertical and lateral circulation passages which, when aligned with ports formed in a packer, permit service fluid such as acids, polymers, cements, sand or gravel laden liquids to be injected into a formation through the bore of the work string and into the outer annulus between a sand screen and a perforated well casing. The annular gravel pack prevents plugging and reduces damage to the screen caused by penetration of formation sand.
In one position of the service seal unit, the annulus below the packer is sealed and the lateral flow passages of the service seal unit are positioned for discharge directly into the annulus between the work string and the well casing, thereby permitting reverse flow and out circulation of clean-out fluids upwardly through the bore of the work string. After the gravel packing or other treatment is finished, completion fluids are introduced into the annulus to displace the service fluids used during well treatment. The service seal unit and the associated wash tube are then removed from the well.
Because of its high value, it is desirable to recover the completion fluid for use during subsequent operations. Additionally, it is desirable to control the effect of completion fluid pressure on the producing formation.